Re: How to get the pixels' value of a bmp image? (Pelles C)

vk10 · #51 29-Jun-2009, 01:08

Hi Dave...

I have a few concern.

1)For the 24bit bitmap, the databuff is of 3-arrays while it is a double array for the 8-bit. why it so?

2)

Quote:

num_bytes_in_table =
> fheader.offset - sizeof(fheader) - sizeof(iheader);
.
.
.
i < num_bytes_in_table / 4

IN the loop, why was the color table iterated till 256.

Quote:

Since the number of bits per pixel is equal to 8, there is a color table.

In 24-bit bitmap, will there not be a color table.

Pls resolve my doubt, Dave

Cheers

davekw7x · #52 29-Jun-2009, 08:53

Quote:

Originally Posted by vk10

1)For the 24bit bitmap, the databuff is of 3-arrays while it is a double array for the 8-bit. why it so?

Are you referring to this from post number 38 in this thread?

Quote:

Originally Posted by davekw7x

Actually, I made them doubles so that I could conveniently round the value:

CPP / C++ / C Code:

    grayvalue = (g + b + r)/3.0 + 0.5;

As I tried to indicate, If g and b and r are chars, then the value of the sum (g + b + r) can be larger than 255, and making them doubles eliminates the possibility that overflow result in the wrong value of the average.

If g, b, r and grayvalue are all unsigned chars, then the same result could be obtained with

CPP / C++ / C Code:

    grayvalue = ((int)b + (int)g + (int)r)/3.0 + 0.5;

or some such thing.

Quote:

Originally Posted by vk10

IN the loop, why was the color table iterated till 256.

As I noted in post 49, the color table in the file may have as many as 1024 bytes for the pixel intensity values (256 integer values, the way that I used it.).

Quote:

Originally Posted by vk10

In 24-bit bitmap, will there not be a color table.

Yes, that's why the routine for processing the 24-bit color bitmap is different from the routine that processes the eight-bit one. So what's the question?

Regards,

Dave

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